This project aims to develop radioligands for s receptors, and to elucidate functions of these receptors, which have been implicated in a various biological processes. [H-3]-Ifenprodil binding demonstrated saturability and high-affinity. Competition assays and studies of and neuroanatomical and subcellular distribution indicate that at 4 degrees C the radioligand labels polyamine binding sites on the N-methyl-D-aspartate (NMDA) receptor but that at 37 degrees C, it binds s receptors. The pharmacological profile of [H-3]ifenprodil binding is highly correlated with that of s-2, but not s-1 sites. Because [H-3]ifenprodil labels s-2 sites, isomers and analogues of ifenprodil were compared as potential s-2 ligands. Of the compounds tested, threo- and erythro-ifenprodil have the highest affinity for s-2 sites. Threo-ifenprodil, which has less affinity for a1-adrenergic receptors than erythro-ifenprodil, is more selective than erythro-ifenprodil for s-2 sites. These results identify threo-ifenprodil as more useful for studies of s-2 receptors than other compounds. 4-Phenyl-1-(4-phenylbutyl)piperidine (4-PPBP) had been identified as a s ligand with high affinity. Studies of [H-3]4-PPBP demonstrate that it is one of the highest affinity s ligands described to date, and that it is potentially useful as radioligand for in vivo labeling of cerebral s receptors. Because previous data have suggested that drugs which interact with s receptors may afford neuroprotection in cases of transient ischemia, we assessed whether 4-PPBP would decrease brain injury from transient middle cerebral artery occlusion (MCAO). Halothane-anesthetized, cats underwent left MCAO and by reperfusion. Cats given 1mmole/kg/h 4-PPBP (from 74 min of MCAO to 4 h reperfusion) had a smaller injury volume in the ipsilateral hemisphere, inferior temporal cortex, and lateral temporal-parietal cortex than cats given saline or low 0.1 mmole/kg/h 4-PPBP. The results suggest that s receptors may mediate neuroprotection in a model of temporary focal ischemia.